I'v been training all my life, basically everything you can train. Just trained tennis 15-20 hours a week for 11 months + did the gym and i mean, i hit the gym for power as well but mostly just making sure im able to practice and play full time without getting injuried + played few futsal games + worked on top of that. Those were the wake up 5am to get home at 7pm days. Didn't get injuried once, few times little joint infections by just simply training too much for my age (31) but got through it. My body breaks down if i skip one of those maintaining sessions on the gym. My rotator cuffs will tear off, i'll probably get tennis elbow, patellar tendonitis etc if i don't do the shit at the gym. It ain't coming for free to swing 360gram racquet for that many hours a week while hitting 200kmh serves, while playing pretty much indoor surfaces that are rock hard instead of grass. That takes its toll and Jarkko Nieminen ex-finnish pro said he trained twice a day, but did the body maintaining 3x a day. Tennis is just terrible for the hips etc. All those wide stances, rough surfaces, heavy rackets etc.
And that's just one season out of many i'v been doing past 31 years.
If pro athlete gets injuried on training and is being injuried alot, there's definitely something wrong in there. You're supposed to have all the physios, all the programs, all the stuff being in front of you to be done. If ex-tennis pro does body maintaining 3x a day, what an average football player does? Prob at the training center all the mandatory, but nothing extra on top of that. Effort is least and injuries are picking up, especially if you're really having big muscular imbalances around your body. But whatever, you get paid the same so it doesn't matter like in individual sports, where every flight you take is from your own pocket. Every injury you got, means no income. That's the difference.